1. Field of the Invention
The invention is generally related to the load out, transportation, and installation of offshore platform decks and more particularly to the installation of a deck onto a floating offshore structure.
2. General Background
There are several methods for installing decks on offshore platforms that are well known in the industry. By far, the most common method is to build the deck onshore in a fabrication yard, lift or skid the deck onto a transport barge, transport the deck to the site on a transport barge, and lift the deck from the transport barge onto the platform substructure using a derrick barge. This is the only method that has been used to install a deck onto a spar type structure. A spar type structure may be a deep draft caisson such as that described in U.S. Pat. No. 4,702,321 or an open or truss framework such as that described in U.S. Pat. No. 5,558,467. A derrick barge is a barge with a revolving crane built into its hull. Ideally, the derrick barge that is available can make a one piece lift of the deck, so that costly hook up work offshore can be avoided. Hook up involves the connection between two or more deck units of structural, piping, electrical, and control systems. If the deck is too heavy for available equipment to lift it from the fabrication yard onto the transport barge, then the deck will be skidded along skid ways onto the transport barge in an operation know as a skidded load out.
In an effort to avoid offshore hook up work, the industry has developed methods other than lifting to install one piece decks. One or more of the alternative methods may be considered whenever a derrick barge of sufficient capacity is not available to make a one piece deck lift.
One of these methods, disclosed in U.S. Pat. No. 5,403,124, includes using a vessel having one end that is U-shaped in plan view. The deck is supported on the vessel over the U-shaped end. The vessel is then moved into position such that the U-shaped end surrounds the platform and the deck is over the offshore platform. The vessel is then ballasted down to transfer the deck onto the floating offshore platform. The width of the U-shape at the end of the vessel limits the maximum size offshore platform on which a deck can be installed by this method. Such a vessel has not been built and this method has not been used.
For a TLP (tension leg platform), the shallow draft of the structure allows it to be brought inshore to relatively shallow and protected water. This allows the deck to be built on the structure and the structure then towed to the installation site after completion.
Spar structures are typically deep draft structures that are six hundred to seven hundred feet tall and thus are incapable of being brought inshore into shallow, protected waters.
It can be seen that for spar structures, there is a need for an alternate method and apparatus for deck installation to that presently available. This need also applies in situations where the floating offshore structure and deck structures are built at different locations and it would be impractical to transport one or both to the same inshore site for installation of the deck onto the floating offshore structure.